Ceramic coatings can be prepared by thermal or plasma spraying and physical vapour deposition (PVD) techniques. In plasma spray, a bulk powder is passed through a plasma and directed towards a substrate where it cools on contact. Films up to 10 mm thick can be produced in this way but usually need post-deposition heating as the deposited film tends to be porous (10% porosity is considered good). Another drawback of the plasma spray technique is that only line of sight geometries can be successfully coated. In PVD, expensive vacuum systems are required to coat high quality ceramic films of less than 10 .mu.m. In addition this technique is also limited to line of sight geometries.
An alternative method, which has gained considerable ground and credibility in recent years, is sol-gel processing. Organo-metallic precursor compounds of the desired ceramic oxides are mixed and dissolved in a suitable solvent. The resultant solution is then hydrolysed to form a structured solution or gel containing organo-metallic polymers or macroclusters. Additives can be added to control the viscosity and surface tension of the sol gel solution. Films are prepared by either spin, dip or spray coating or painting onto an appropriate substrate. The coated substrate is then fired to remove the organic material and a post-fire heating step is usually performed to fully develop the final ceramic structure. The sol gel process has several advantages over other fabrication methods. It is simple, more economically feasible and permits coating of complex geometries, not necessarily line of sight. Usually ceramic films up to about 0.5 .mu.m can be deposited in a single layer but films up to about 3.0 .mu.m have been produced using a complex vacuum controlled firing treatment. Thicker films, up to about 10 .mu.m in thickness have been produced by successive coatings in 0.1 .mu.m layers. Clearly a 10 .mu.m film made 0.1 .mu.m at a time is a very time consuming and laborious process. In order to exploit the desired properties of the ceramic, it is essential that the ceramic film should be crack-free. Sol Gel films are, however, very susceptible to substrate interaction, defects and stresses within the film. Generally the thinner the film the lower the internal stresses and the number of defects. Solvents and organics can escape from 0.1 .mu.m layers relatively easily but for thicker layers this is not so resulting in defects which can act as nucleation centres for crack propagation. Sol gel films are also substrate dependent and most films are limited to metallized silicon or other highly polished substrates. Conventional sol gel technology cannot, therefore, be used to produce thick, large area sol gel films.